The present invention relates generally to optical filters, and more particularly to an optical spatial filter for filtering spatial frequencies from an optically formed image.
Spatial filtering is routinely implemented in optical systems to decrease error in the corresponding image. Such error is primarily encountered in an image which has been sampled, resulting in a harmonic duplication of the original spectrum in the spatial frequency domain. Appropriate spatial filtering reduces this error. Optical spatial filtering is directly related to modifying the Point Spread Function (PSF) of the optical system. Therefore, modifying the PSF of an optical system by proper spatial filtering, can similarly reduce image error.
Historically, spatial filters were applied in the form of an opaque mask consisting of transparent or partially transparent apertures. However, such filters are limited in utility due to significant loss of light. In the last thirty years, transparent, phase-only spatial filters have been developed which do not suffer from this loss. These filters are generally comprised of a plurality of transparent subapertures on a transparent substrate wherein the relative thickness of subapertures is designed to adjust the phase composition of the light passing through the optical system. Such filters are generally impractical to fabricate, are quite sensitive to color variations and also produce a significant blurring of the image.
Other known optical low-pass filter arrangements have utilized a birefringent plate to refract the incident light into two almost overlapping images. However, birefringent plates are inherently very sensitive to polarization, wavelength variation and direction of the incident light. Also, birefringent plate filters have an attendant high cost due to the materials, e.g. quartz, used to construct the filter.
In U.S. Pat. No. 5,142,413, entitled "Optical Phase-Only Spatial Filter," incorporated by reference herein, a new type of transparent spatial filter is taught utilizing a plurality of subapertures which provide filtering by destroying spatial coherence. Such a forth is much more practical to fabricate than the previous forms and is much less sensitive to color. However, the above described filter still produces significant image blur due to the inherent filtering effect as well as to scattering by the filter structure. Further, the difficulties associated with the above forms of spatial filters restrict their optimum location in the optical system to regions significantly removed from the image plane, primarily due to increased scattering in the associated diffraction-based designs as the image plane is approached.
The most common use of a spatial filter is to broaden the PSF of an optical system--increasing the size of the smallest resolvable detail. Image blur is produced by current spatial filters because such spreading cannot be entirely confined to a desired spot size.